The Truth about CMR, Part 3 - Digestion
28 February 2018
The calf’s ability to digest sugars, fats and proteins in the abomasum and duodenum are pH dependent.
In the third part of his investigation into the whys and wherefores of calf nutrition, Jim Uprichard turns his attention to the actual science behind the digestive process of the young animal...
Protein digestion by protease enzymes occurs in the abomasum and efficient digestion requires an acidic environment. The pH is more alkaline in the upper part of the small intestine, where sugars and fats are digested. During the first weeks of life there is little or no starch digestion in the intestinal tract. To maintain the pH of the intestinal tract, the calf secretes electrolytes or mineral salts along with enzymes.
For rapid growth in the calf’s first few weeks of life, the calf milk replacer must be made up of highly digestible nutrients that fully digest before they reach the hind gut. Otherwise, fermentation of nutrients and scouring can occur, as the calf excretes the fermentation acids that are produced.
Development of the rumen requires structural fibre (from straw) which provides physical bulk to increase volume and stimulate muscle development. Rumen development also requires digestible fibre, carbohydrates and protein (from starter concentrates) to encourage rumen microbial growth. The by-products of microbial carbohydrate fermentation are acetic acid (mainly from fibre digestion), and propionic and butyric acid (mainly from starch and sugar fermentation). Acetate and propionate are absorbed through the rumen wall as energy sources, while butyrate is used as an energy source for rumen papillae growth.
As the rumen papillae become functional, the ability to absorb VFA’s increase and rumen pH increase as. Until the rumen stabilises at pH 6 or higher (which typically takes most of the first 10 weeks of life), cellulolytic bacteria are unable to thrive. Therefore, fibrous feeds should not make up the majority of the diet during this time.
A correct level of rumen fermentable protein is required to supply a nitrogen substrate for the rumen bacteria to multiply. Excess nitrogen can cause issues, as it will either be not be used and processed by the liver to urea for excretion, or end up in the immature abomasum where the lack of sufficient protease enzymes means it can be undigested and cause problems further down the digestive tract.
Digestion of Calf Milk Replacer Ingredients
Both the amount and types of ingredients that can be used in calf milk replacer without compromising growth or health are relatively limited; digestive enzymes present at birth and during pre-ruminant phase only allow for highly effective digestion of milk proteins, lactose and fats (triglycerides). Young calves (pre-ruminant calves) have much less ability to digest non-milk protein or polysaccharides such as starch.
Whole milk bypasses the rumen via closure of the oesophageal grove and enters the abomasum.
Skimmed milk powder (SMP) also enters the abomasum via the same principle as whole milk. In the abomasum, the acidic conditions (pH around 2) begin the breakdown of casein protein in SMP. The inactive enzyme prorennin (produced in the abomasal wall) is activated by the acid conditions in the abomasum to the enzyme rennin. In the presence of calcium ions, rennin cleaves a specific peptide bond in casein, causing coagulation of casein protein to form a curd.
Fat is trapped in the curd, whereas whey protein, lactose, soluble minerals and vitamins pass directly into the small intestine within 2 to 3 hours after feeding. Therefore, the casein protein in SMP is digested slower than whey protein.
Whey milk powder (WMP), also enters the abomasum via the oesophageal grove however, as a milk curd is not formed in the abomasum, whey proteins are passed into the small intestine shortly after the feed. In the acid conditions of the abomasum, the enzyme pepsinogen is activated to pepsin, which partially digests casein to polypeptides before they are released into the small intestine. In the small intestine, whey proteins and polypeptides are digested by the pancreatic enzymes trypsin, chymotrypsin, carboxypeptidase and elastase. Peptidase on the intestinal brush border completes the breakdown of peptide proteins.
Free amino acids, dipeptides and tripeptides are absorbed across the intestinal wall via specific transport proteins. It is often claimed that skimmed milk calf milk replacers result in improved performance and ‘coat bloom’ in calves. However, research at Harper Adams University College in the UK, showed no significant difference throughout the entire rearing period, between calves reared on skimmed milk powders or whey based powders (Marsh and Boyd, 2011). These findings support the earlier research that shows similar calf performance and health in comparative studies between the two milk protein sources (Terosky et al., 1997) (Lammers et al., 1998).
As whey-based calf milk powder passes into the small intestine sooner, it is important that only the most digestible ingredients are included to avoid nutritional scours that may occur when undigested nutrients ferment in the large intestine.
A young calf’s digestive system is adapted to the digestion of milk-proteins rather than non-milk proteins. Digestion of many non-milk proteins (for example, soya) is very limited in the first 3 to 4 weeks of the calf’s life. Milk protein have been shown to achieve superior growth and health performance compared to non-milk proteins.
The enzyme ‘pregastric lipase’ is present in saliva, and this starts the digestion of milk fat (triglycerides) in the abomasum, by breaking down triglycerides in the milk curd to diglycerides and free fatty acids. Fatty acids have different structures, and are comprised of short-, medium- and long-chain fatty acids, which determine how it is digested and absorbed.
The pregastric lipase in saliva will break down short and medium chain fatty acids to butyrate and other short chain fatty acids, which are absorbed in the small intestine and are used as an energy source for the calf. Medium chain fatty acids (8 to 10 carbons in length) have antimicrobial properties and create an unsuitable environment for pathogens in the small intestine.
The pancreas also produces lipase to digest fat, but not until the calf is older than two weeks of age. In the small intestine (in the presence of bile salts and colipase), pancreatic lipase breaks down diglycerides and any remaining triglycerides, to 2-monoglycerides and free fatty acids. 2-monoglycerides and bile salts form micelles, which allow lipids to be absorbed into the intestinal epithelium cells. They are then reconverted to triglycerides, before being packaged into lipoproteins to be picked up by the lymphatic system.
As the milk sugar, lactose, does not form part of the milk curd, it rapidly passes into the small intestine where the enzyme lactase splits it into the single sugars, glucose and galactose. These single sugars are absorbed across the intestinal epithelium via specific protein transporters.
During the first 3 to 4 weeks of life, lactase enzymes predominate, meaning the calf can effectively use lactose. Lactose is digested quicker than fat, while calves lack the enzyme sucrase to digest sucrose.
The starch in starter concentrates is an important energy source for older calves. Young calves, particularly those less than 2 weeks old, cannot efficiently digest starch as the activity of amylase and maltase enzymes are low. As the calf grows and the rumen develops, starch can then be fermented to produce (predominately) propionic and butyric acid.
The intake of starch intake is important for rumen development as butyric acid provides energy for rumen wall development.